Photographic silver halide emulsions are generally prepared by precipitation of the silver halide in the presence of a carrier or binder, generally gelatin, wherein the silver halide crystals are formed by the interaction of a water-soluble silver salt, such as silver nitrate, and a water-soluble halide, such as potassium bromide. The term "crystal" as used herein refers to a crystalline particle of silver halide, sometimes referred to as a grain, and should be understood to include particles of any composition of silver halide with any mixture of crystal habits. Subsequent to the precipitation, the mixture is heated for a given period of time. Additional binder may be added at this point. The binder-silver halide mixture is then generally chilled, noodled and washed to remove the soluble salts. The mixture may again be melted, chilled and washed if desired. Various substances such as sensitizers, coating aids, and the like, may also be added to the emulsion during its preparation, generally after the washing stage, and a heat treatment may be applied to induce the process known as chemical sensitization.
From the foregoing, it will be noted that the formation of the silver halide crystals and the sensitization takes place in the presence of a binder material. Gelatin is the most commonly used binder material for silver halide, but other materials such as synthetic polymers are also employed. It is a requirement of the binder material that it permit the growth of silver halide crystals but that the growth not be too rapid to prevent crystals of undesirable size from being formed. It is also a requirement that the binder material allow the various sensitization processes to take place. These requirements disqualify a large number of synthetic polymeric materials from being employed in silver halide emulsions when otherwise they may possess other properties desired in such employment. In addition, because the reaction forming the silver halide grains takes place in the presence of the binder, starting materials are entrapped therein which require careful and extensive washing procedures.
It is also known that silver ions can form water-soluble complex ions with numerous inorganic and organic compounds. Specific silver halide complexes include the AgCl.sub.2.sup.-, AgCl.sub.4.sup.-3, AgBr.sub.3.sup.-2, AgBr.sub.5.sup.-4, Ag.sub.2 Br.sup.+, AgI.sub.2.sup.-, AgI.sub.4.sup.-3, AgCl.sub.3 Br.sup.-3, Ag.sub.3 I.sup.+2 and AgClBr.sub.3.sup.-3 complexes. It is believed that the foregoing complexes participate in the growth of the silver halide crystals during physical ripening of the emulsion and influence the kinetics of development. The above-mentioned complexes are known to the art and are set forth on page 8 of Mees, The Theory of the Photographic Process, Third Edition, MacMillan Company, New York.
It is with the above-described silver complexes that the present invention is concerned.